Plate heat exchangers typically consist of two end plates in between which a number of heat transfer plates are arranged in an aligned manner, channels being formed between the heat transfer plates. Two fluids of initially different temperatures can flow through every second channel for transferring heat from one fluid to the other, which fluids enter and exit the channels through inlet and outlet port holes in the heat transfer plates.
Typically, a heat transfer plate comprises two end areas and an intermediate heat transfer area. The end areas comprise the inlet and outlet port holes and a distribution area pressed with a distribution pattern of projections and depressions, such as ridges and valleys, in relation to a reference plane of the heat transfer plate. Similarly, the heat transfer area is pressed with a heat transfer pattern of projections and depressions, such as ridges and valleys, in relation to said reference plane. The ridges of the distribution and heat transfer patterns of one heat transfer plate is arranged to contact, in contact areas, the valleys of the distribution and heat transfer patterns of another, adjacent, heat transfer plate in a plate heat exchanger. The main task of the distribution area of the heat transfer plates is to spread a fluid entering the channel across the width of the heat transfer plate before the fluid reaches the heat transfer area, and to collect the fluid and guide it out of the channel after it has passed the heat transfer area. On the contrary, the main task of the heat transfer area is heat transfer.
Since the distribution area and the heat transfer area have different main tasks, the distribution pattern normally differs from the heat transfer pattern. The distribution pattern is such that it offers a relatively weak flow resistance and low pressure drop which is typically associated with a more “open” distribution pattern design, such as a so-called chocolate pattern, offering relatively few, but large, contact areas between adjacent heat transfer plates. The heat transfer pattern is such that it offers a relatively strong flow resistance and high pressure drop which is typically associated with a more “dense” heat transfer pattern design, such as a so-called herringbone pattern, offering more, but smaller, contact areas between adjacent heat transfer plates.
The locations and density of the contact areas between two adjacent heat transfer plates are dependent, not only on the distance between, but also on the direction of, the ridges and the valleys of both heat transfer plates. As an example, if the patterns of the two heat transfer plates are similar but mirror inverted, as is illustrated in FIG. 1a where the solid lines correspond to the ridges of the bottom heat transfer plate and the dashed lines correspond to the valleys of the top heat transfer plate, then the contact areas between the heat transfer plates (cross points) will be located on imaginary equidistant straight lines (dashed-dotted) which are perpendicular to a longitudinal center axis L of the heat transfer plates. On the contrary, as is illustrated in FIG. 1b, if the ridges of the bottom heat transfer plate are less “steep” than the valleys of the top heat transfer plate, the contact areas between the heat transfer plates will instead be located on imaginary equidistant straight lines which are not perpendicular to the longitudinal center axis. As another example, a smaller distance between the ridges and valleys corresponds to more contact areas. As a final example, illustrated in FIG. 1c, “steeper” ridges and valleys correspond to a larger distance between the imaginary equidistant straight lines and a smaller distance between the contact areas arranged on the same imaginary equidistant straight line.
At the transition between the distribution area and the heat transfer area, i.e. where the plate pattern changes, the strength of the heat transfer plate may be somewhat reduced as compared to the strength of the rest of the plate. Further, the more scattered the contact areas are at the transition, the worse the strength may be. Consequently, similar but mirror inverted patterns of two adjacent heat transfer plates with steep, densely arranged ridges and valleys typically involves a stronger transition than differing patterns with less steep, less densely arranged ridges and valleys.
A plate heat exchanger may comprise one or more different types of heat transfer plates depending on its application. Typically, the difference between the heat transfer plate types lies in the design of their heat transfer areas, the rest of the heat transfer plates being essentially similar. As an example, there may be two different types of heat transfer plates, one with a “steep” heat transfer pattern, a so-called low-theta pattern, which is typically associated with a relatively low heat transfer capacity, and one with a less “steep” heat transfer pattern, a so-called high-theta pattern, which is typically associated with a relatively high heat transfer capacity. A plate pack containing only low-theta heat transfer plates will be relatively strong since it is associated with a maximum number of contact areas arranged at the same distance from the transition between the distribution and heat transfer areas. On the other hand, a plate pack containing alternately arranged high-theta and low-theta heat transfer plates will be relatively weak since it is associated with a smaller number of contact areas arranged at the same distance from the transition.
The above problem is described further in applicant's Swedish patent SE 528879 which is hereby incorporated herein by reference and which also discloses a solution to this problem. The solution involves the provision of a narrow band between the distribution and heat transfer areas of the heat transfer plates irrespective of plate type. The narrow band is provided with a herringbone pattern, more particularly densely arranged “steep” ridges and valleys. Thereby, the transition to the distribution area will be the same and relatively strong irrespective of which types of heat transfer plates the plate pack contains.
However, even if the narrow band above solves the strength issue at the transition to the distribution area, it occupies valuable surface area of the heat transfer plates without being associated with either effective fluid distribution due to the density of the ridges and valleys, or effective heat transfer due to the “steepness” of the ridges and valleys. More particularly, the heat transfer capacity of the narrow band is relatively low as compared to the heat transfer capacity of a heat transfer surface of a high-theta heat transfer plate. However, the heat transfer capacities of the narrow band and the heat transfer surface of a low-theta heat transfer plate may be about the same.